Relative velocities can be determined by means of a Doppler radar. Emitted radar waves are reflected off of the object which is to be measured. The measured frequency of the reflected radar wave is diferent from the frequency of the emitted radar waves owing to the Doppler effect. This diffe- rence is in proportion to the relative velocity between the unit which sends and receives radar waves and the object which is to be measured and reflects the radar waves. The frequency change is obtained as a beat frequency by merging the emitted and the reflected signals. This frequency is designated as a "Doppler frequency." A laser can also be used instead of a radar. In this way, a Doppler frequency associated with the relative velocity is also obtained in a similar fashion. Another possiblity for measuring the relative velocities consists in measuring the Doppler frequency of sound waves emitted by an acoustic transmitter.
"Doppler modules" are commercially available components and are, for example, described in a company publication PS 6043 Issue 1 10 80 "Plessey Oscillators and Doppler Modules for Industrial and Commercial Use" published by Plessey Opto-electronics and microwave Ltd.
Various arrangements of such Doppler modules for room monitoring as "photoelectric barriers" or for the automatic control of gates are also described in the mentioned publication. Furthermore, this publication also describes the use of such a Doppler module for measuring velocity.
The previously mentioned Plessey publication further describes a Doppler module with two mixer diodes which supply Doppler signals phase shifted by 90.degree.. Not only can the velocity of a reflecting object relative to the Doppler module be determined from these Doppler signals but the direction of the movement as well. Approaching and receding objects can thus be differentiated.
Doppler modules using radar are used for monitoring traffic in order to determine if the speed limit is exceeded, see, for example Swiss Patent No. 662 660.
During use, this Doppler module is arranged in a stationary fashion. The backgrou nd and surface do not produce a Doppler frequency. The Doppler frequency only originates from moved objects, e.g. from the motor vehicles which are to be monitored.
Through "Funkschau" 1974, Vol. 5, pages 1955 to 1956, an apparatus is known which is intended to prevent collisions between motor vehicles and obstacles with the help of a radar device and a signal processor. Using this known apparatus, the vehicle's distance from the obstacle and the vehicle's velocity relative to the obstacle and to the road is measured by means of the radar device. Depending on the stored values which relate to the assumed road condition, and with the help of a decision circuit, a criterion is formed to test if the vehicle approaches the obstacle at a dangerously high speed.
In doing this, it is necessary that the radar device virtually targets such an obstacle only and not the other stationary elements of the background and surface. That excludes, though, a long-range radar beam. Also, with a closely bunched radar beam, a false alarm is triggered when the vehicle drives through a curve where objects which reflect the radar signal, e.g. trees, houses, warning sign posts, lampposts, or crash barriers, stand along the side of the road.
From German Patent No. 2 654 660 there is known a method for preventing collisions between road vehicles and obstacles. Therein, the position of the vehicle's steering wheel is additionally adjusted in order to avoid false alarms. The vehicle's velocity and the steering wheel position form addresses and define a memory location of a memory unit subdivided into lines and columns. A set of input variables is stored at this memory location. A comparator compares the distance detected by the radar device with a minimum distance read out from the memory unit. Such an arrangement is complicated. It requires a radar distance measurement. It does not detect the case where, for example, the vehicle drives in a straight line towards a curve without the vehicle's steering wheel first being turned. It is also assumed here that the radar device normally only detects a potential obstacle but not the remaining background and surface, e.g. the road surface or roadside trees.
German Published Patent Application No. 2 558 144 which is cognate with the Japanese Published Patent Application Number 50-1561, relates to a method for cutting out undesired echo signals which are reflected by false targets in a controlled zone with the emission of a radar beam from a motor vehicle as it passes the controlled zone. To this end, markers are fixed at the entrance and exit of the controlled zone. The markers at the entrance are scanned when the motor vehicle drives into the controlled zone. Due to this scanning operation, the scanning range of the signals reflected by stationary objects in the controlled zone is reduced. When the motor vehicle drives out of the controlled zone the markers at the exit are scanned. The scanning range is thereby returned to its normal value.
This method can also be used for traffic monitoring with a stationary radar device.
Through the company publication MAX 280/LTC 1062 of the SE Spezial-Electronic KG, Kreuzbreite 14, D-3062 Buckeburg,, Germany, a fifth-order low-pass filter without offset voltage in the form of a chip is known. The frequency limit of this filter is determined by internal clock pulses which can be overridden by external clock pulses. Such low-pass filter enables the frequency limit of the filter to be altered depending on an external signal. National Semiconductor of Santa Clara, Calif., make switched capacitor filters of the type MF4 and MF6 which also allow the upper frequency limit to be determined by external clock pulses. The MF4 and MF6 devices are described in the National Semiconductor Data Acquisition Handbook.
Through a catalogue from Motorola, No. MC 14046 B, a phase locked loop designed as an integrated circuit is known. Such phase locked loop has two phase comparators, a voltage-controlled oscillator, a current source-follower and a Zener diode. The phase comparators compare two signal inputs in the form of frequencies. The first phase comparator, an exclusive OR-element, supplies a digital actuating signal and maintains a phase shift by 90.degree. at the medium frequency between the two signal inputs. The second phase comparator has a logic unit reacting to the leading edge, and supplies complementary digital actuating signals and maintains the signals at the two signal inputs in phase. The linear, voltage-controlled oscillator produces an output signal whose frequency is determined by an applied voltage.
Through a company publication "TMC 2310" of TRW LSI Products, Inc., a processor is known by which a Fourier transform of an input signal progression can be quickly determined and stored (FFT=Fast Fourier Transform).